Skid-resistant pavement markings

ABSTRACT

A skid-resistant pavement marking sheet including a top surface and a plurality of skid-resistant ceramic spheroids embedded in and protruding from the surface. The ceramic spheroids are fired from a raw material mixture including 10-97 wt. % mineral particulates, 3-90 wt. % alumina, and at least 1 wt. % binder, at a temperature of about 1300° C., have an outer surface rich in alumina concentration relative to the center of the spheroid, and are resistant to crushing and fracturing.

BACKGROUND OF THE INVENTION

The present invention relates to the use of particles to preventskidding and in particular to the prevention of skidding on pavementmarking sheets.

Pavement marking sheets or tapes are well known to convey information todrivers of motor vehicles and pedestrians. The marking sheets typicallyhave a highly visible color and often employ retroreflective particlesfor nighttime use.

Representative of pavement-marking sheet materials are U.S. Pat. No.4,117,192 to Jorgensen (also disclosing irregular skid-resistantmaterials); U.S. Pat. No. 4,248,932 to Tung (disclosing skid-resistingirregular or angular inorganic particles, and optionally retroreflectivespheres, scattered over the surface as being more skid-resistant thandense packed particles. The disclosed sheet material has askid-resistance of at least 55 BPN); and U.S. Pat. No. 4,758,469 toLange (disclosing an embossed retroreflecting pattern on a marking tapewith optional embedded skid-resisting particles.

Anti-skid properties are desirable on many surfaces and are particularlydesirable in pavement marking tapes in order to reduce slipping bypedestrians, bicycles, and vehicles upon the tapes. Several Europeangovernments have specified desirable levels of skid-resistanceproperties for pavement marking sheet materials. The specifications callfor a minimum level of initial skid-resistance and retention of asignificant level of skid-resistance "in use." When installed upon apavement surface, (ie. "in use"), the marking tapes are subjected to anenvironment which is quite severe.

In the pavement marking industry, skid-prevention properties aretypically imparted by embedding hard, crystalline particles with sharppoints on an upper surface of the pavement marking tape. Typicalexamples of such conventionally employed particles include corundum(aluminum oxide) and quartz (sand, silicon dioxide, or micronizedquartz). These particles are capable of achieving relatively goodinitial skid-resistance, however, the initial skid-resistance begins todecrease rapidly once the marking tape is exposed to traffic.Specifically, the impact of tires, the abrasion of loose dirt and sand,and the corrosion of salt and water contribute to loss ofskid-resistance. The loss of skid-resistance is typically caused by somecombination of at least one of two mechanisms. First, the crystallinematerials have a tendency to fracture along crystalline planes. Second,the particles may become loosened from the embedding matrix.

One type of pavement marking tape, which employs a tough, durable toplayer which resists wear or ablating and includes partially embeddedcrystalline particles to impart skid-resistance is disclosed in U.S.Pat. No. 4,020,211 to Ludwig Eigenmann. The Eigenmann product is apavement marker with a plurality of particles bonded to a layer andprojecting outwardly. The particles have a minimum hardness of about 6on the Mohs Hardness Scale and include a pointed end portion projectingoutwardly for imparting good anti skid properties. However, poorretention of acceptable skid-resistance in use has been observed.

An alternative product attempts to counter the above mechanisms byproviding an ablative pavement marking sheet which has crystallineparticles distributed throughout the thickness of the marking sheet,thereby continuously exposing fresh skid-resistant particles as themarking sheet wears. One drawback to the ablative marking tapes ismaintaining an acceptable and informative color "in use," since ablativematerials are typically soft and tend to become embedded with dirt. (SeeU.S. Pat. No. 4,490,432).

A type of particle, previously unknown to the pavement marking industryis disclosed in U.S. Pat. No. 4,680,230. The ceramic particles aretaught to be useful as proppants in oil well hydraulic fracturing toimprove pumping production. The particles comprise a vitreous matrixphase containing a crystalline alumina phase. The particles arecharacterized by a Krumbein roundness of at least 0.8 (1.0 representinga perfect sphere) and are highly chemically stable.

Canadian Patent No. 1002803 discloses an iron paving slab with recessesinto which a binder and pulverized material, such as a ceramic iscoated.

Japanese Patent No. 60130660 discloses a melt application type non-slipmaterial which is filled with ceramic aggregate of baked silica,alumina, or clay and pigments.

Swiss Patent No. 541674 another Eigenmann patent discloses the use ofglobules of a binding material containing corundum (Aluminum Oxide).

West German Patent No. 2927362 discloses a material for abrasionresistant surfaces comprising glass powder embedded in a polymer matrix.

SUMMARY OF THE INVENTION

We have discovered that spheroidal, crush resistant ceramic particlesoriginally developed for use as proppants in the hydraulic fracturingmethod of increasing oil well production impart improved skid-resistancewhen partially embedded in a surface of a matrix layer. The particlesalso demonstrate excellent particle retention in comparison withconventional pointed skid-resistance particles presently employed in thepavement marking industry. Further, the use of these particles allowsthe surface to retain a surprisingly high level of skid-resistance overa long period of time.

Our discovery of a novel use for these spheroidal particles as anti-skidparticles allows the production of highly skid-resistant pavementmarking sheets. The sheets include a matrix layer and a plurality ofcrush resistant ceramic particles partially embedded in and protrudingfrom the matrix layer. The skid-resistant sheets retain a high level ofskid-resistance while exposed to severe road environment conditions fora surprisingly long time.

Additionally, the spherical particles can be custom colored or coated tomatch the color of pavement marking tape.

In one aspect, the present invention concerns a durable, skid-resistantsurface, which is useful as a pavement marking sheet material, thesurface including a plurality of substantially spherical, crushresistant particles partially embedded in a matrix layer.

In another aspect, the present invention concerns a method of producinga durable skid-resistant sheet material.

In yet another aspect, the present invention concerns a method toprevent skidding by embedding spherical ceramic crush resistantparticles in a matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a pavement marking sheet of thepresent invention.

FIG. 2 is a graph of skid-resistance of a pavement marking tape of thisinvention and a comparative tape of sand over extended time.

FIG. 3 is a graph of skid-resistance of 2 pavement marking tapes of thisinvention and 2 tapes of conventional skid-resistant particle over time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In an effort to enhance both the skid-resistance and durability ofskid-resistance of pavement marking sheet materials, a number ofparticles were tested as alternatives to typically employed highlypointed crystalline particles such as "Sand Saba Sand." One of theparticles tested, a spheroidal, crush resistant ceramic particledemonstrated surprisingly superior skid-resistance and durability ofskid-resistance. The particle is described in U.S. Pat. No. 4,680,230,which is incorporated herein by reference. The spheroidal particlestypically have a Krumbein roundness of at least about 0.8. Atheoretically perfect sphere is defined as having a Krumbein roundnessof 1.0. The particles usually range in size from about 0.3 mm to about2.0 mm in diameter, more typically from about 0.4 mm to about 1.2 mm indiameter, and preferably from about 0.4 mm to about 1.0 mm in diameter,although alternative screening processes may be employed to produceother ranges of particles diameters.

The particles include a high alumina content. The porous core istypically a blend of from 10 to 90% wt. Alumina, preferably a 50--50blend of crystalline Al₂ O₃ and vitrified mineral fines, with a higherconcentration of alumina near the surface of the spheroid than in thecenter.

The ceramic particles allow for the production of pavement marking sheetmaterial with surface frictional properties of at least about 45 BritishPendulum Number (BPN) as determined by ASTM E303 when exposed to trafficfor a substantial period of time in the wheel track on a typicalpavement. The particles also may be colored to match the color ofpavement marking tapes. The color coated particles are particularlyadvantageous in that a pavement marking tape including a plurality ofprotruding skid-resistant particles has a more uniform color appearance,which in turn improved the efficiency of an observer in discerning theinformation to be conveyed by the marking strip.

In the crush resistance test described hereinafter, the spheroidalparticles typically have less than about 22 wt. % fractured at anapplied pressure of approximately 69 MPa, compared to Ottawa sand whichtypically has about 41 % wt. fractured at about 69 MPa. The particlesare essentially insoluble in salt water solutions. The Critical StressIntensity Factor, a measure of fracture toughness of the particlesK_(IC), can be approximately twice that of sand, being typically morethan about 1.5 Mn/m^(3/2) compared to about 0.7 for sand. K_(IC), asexplained by B. R. Lawn et al. in J. American Ceramics Soc., Vol. 63, p.574 (1980), is determined by microscopic examination of fracturesoriginating from a diamond imprint on a polished surface of a particleand is calculated as K_(IC) =0.028E^(1/2) H^(1/2) a² c^(-3/2), where "E"is the bulk modulus, "H" is the Vicker's hardness, "a" is the length ofa diagonal of the diamond imprint and "c" is the length of a plus thelength of a fracture originating at a corner of the diamond imprint. TheMohs hardness of the particles is typically 7 or higher, similar to thatof sand.

A pavement marking sheet material incorporating the present invention isgenerally shown in FIG. 1 at 10. The sheet 10 includes a first layer 12,in contact with a pavement surface 14. Optionally, a fine layer ofadhesive material 15 may be interposed between the first layer 12 andthe pavement 14. A second layer 16 is coated upon the underlying firstlayer 12. The second layer 16 has an uppermost or top surface 20. In apavement marking sheet 10, the uppermost or top surface 20 may bedescribed as that surface which would contact a vehicle tire or thebottom surface of a pedestrian's foot.

Protruding from and partially embedded in the uppermost surface 20 ofthe second layer of the marking sheet are a plurality of crushresistant, ceramic spheroids 22 which impart skid-resistance to theuppermost surface 20.

In one alternative embodiment, the uppermost surface 20 may be anembossed or patterned surface and need not be limited to a smooth,continuous surface but rather may be adapted to facilitate rapiddrainage of water. Additionally, the uppermost surface may optionallyinclude retroreflective beads 24 or other equivalent materials commonlyassociated with pavement marking sheet materials.

Suitable material for the second layer 16 may be either a thermoplasticor thermosetting polymeric binder. One such binder is a vinyl-basedthermoplastic resin, including a white pigment, as described in U.S.Pat. No. 4,117,192 incorporated herein by reference. Other suitablematerials for the second layer 16 include two-part polyurethane formedby reacting polycaprolactone diols and triols with derivatives ofhexamethylene diisocyanate; epoxy based resins as described in U.S. Pat.No. 4,248,932, U.S. Pat. No. 3,436,359, and U.S. Pat. No. 3,580,887; andblocked polyurethane compositions as described in U.S. Pat. No.4,530,859. Also suitable as a material for the second layer 16 arepolyurethane compositions comprised of a moisture activated curing agentand a polyisocyanate prepolymer. The moisture activated curing agent ispreferably an oxazolidene ring. Such compositions are described in U.S.Pat. No. 4,381,388.

The preferred polyurethane material is formed by first reacting twoequivalents of methylene bis (4-cyclohexyl isocyanate) (H₁₂ MDI) withone equivalent of a polycaprolactone triol of molecular weight about 540and hydroxyl number about 310 (i.e., a 2-oxypanone polymer with2-ethyl-2-(hydroxymethyl)1,3 propanediol) using dibutyltindilaurate as acatalyst. The reaction is carried out in 2-ethoxyethyl acetate andcyclohexanone. To 25 parts of prepolymer is also added 20 parts of a60/40 pigment dispersion of pigments such as either titanium dioxide orlead chromate in a diglycidyl ether of bisphenol A epoxy resin (asuitable source is Stan-Tone® 10 EPX03 or 30 EPX03 made by HarwickChemical Corp. of Akron, Ohio). Zinc 2-ethylhexanoate catalyst is addedto the liquid mixture shortly before coating or applying the liquidmixture to the first layer 12. Inclusion of up to about 10% 2,4pentanedione in the preferred bead bond extends the pot life of thecoating mixture from about 1.5 hours to about 15 hours.

Useful ranges of pigment dispersion which may be included in the secondlayer 16 are 10-30 parts per 25 parts of urethane prepolymer.Hydrogenated expoxies may also be employed. Other useful pigmentsinclude nacreous pigments (such as lead sulfate) and yelloW iron derivedpigments. Other pigments typically used for coloring pavement markingsmay also be used.

Generally, suitable materials for the second layer 16, such as describedabove, are characterized by excellent adhesion to skid-resistantparticles 22 which are subsequently partially embedded in the secondlayer 16 prior to curing. Additionally, the material of the second layer16 strongly adheres to the base sheet material of the first layer 12, ishighly cohesive and resistant to environmental weathering.

A pavement marking sheet of this invention may be prepared by providinga base sheet, coating a face of the base sheet with a liquid bondingmaterial, embedding the crush resistant spheroids in the liquid bondingmaterial and curing or solidifying the bonding material to embed theparticles in a partially protruding orientation on the marking sheet.The base sheet may be prepared, for example, as previously disclosed inthe Jordan Patent, U.S. Pat. No. 4,490,432. Preferably, a portion ofpolyester fibers, may be included in the mixture. In this method, amixture is formed, then calendared into a sheet or "premix." The premixis subsequently coated with a liquid bonding material, such as aurethane resin including a catalyst and preferably including a pigment,such as TiO₂. A spreading bar is a suitable method of application. Thecrush resistant spheroids are embedded in the liquid bonding material bycascading or sprinking. Subsequently, curing of the bonding material byexposure to moisture and/or heat in an oven, solidifies the bondingmaterial and locks the spheroids in a partially embedded position whichallows skid-resistance properties to be imparted to the surface.

The preferred ceramic spheroids may be prepared as follows: a mixture ofdry raw materials including a mineral particulate and a binder arepelletized in a high energy pelletizing apparatus, by adding water. Theresulting wet product is dried, then mixed with a parting agent.Subsequently, the dried pellets and parting agent are fired to causevitrification to occur.

Machines known as high energy mix pelletizers are best suited for thepellet formation step. Two examples of such machines are the Littlefordmixer and the machine known as the Eirich machine. The Eirich machine isdescribed in U.S. Pat. No. 3,690,622, incorported herein by referenceThe Eirich machine comprises basically a rotatable cylindricalcontainer, the central axis of which is at an angle to the horizontal,one or more deflector plates, and at least one rotatable impactingimpeller usually located below the apex of the path of rotation of thecylindrical container. The rotatable impacting impeller engages thematerial being mixed and may rotate at a higher angular velocity thanthe rotatable cylindrical container itself.

Numerous variation of the above particles have also been prepared andare contemplated as part of the subject invention. For example, 3Mroofing granule mineral fines of approximately 8 um diameter or less maybe substituted for the nepheline syenite fines. The amount of wateremployed in the pelletizing step may be increased to provide somewhatmore irregular shaped particles. The coarse alumina is preferablyemployed as a parting agent during the firing step, however, the coursealumina may be partially or completely replaced by fine calcined aluminaor alumina hydrate.

Particles offered commercially by Carbo Ceramics of New Iberia, La.70560, known as Carbolite and Carboprop are also suitable as durableanti-skid particles in this invention. Suitable particles made of clayand bauxite are also sold commercially by Norton-Alcoa. Another sourceof durable spherical particles which are believed suitable forpracticing the present invention are the particles described in U.S.Pat. Nos. 4,072,193 and 4,106,947 which are monoclinic and cubiczirconium embedded in a modified alumino-silicate glass. Such particlesare available from Messina, Inc. of Dallas, Tex. under the name"Z-prop."

An important parameter for evaluating particles is crush strength orcrush resistance. Means for evaluating the crush resistance of theparticles are found in American Petroleum Institute Publications suchas: "API Recommended Practices for Testing Sand Used in HydraulicFracturing Operations" API RP 56, 1st Edition, (March, 1983) and "APIRecommended Practices for Testing High Strength Proppants Used inHydraulic Fracturing Operations," 3rd Edition, (January, 1983). Crushresistance is measured by placing a sample of particulate material intoa test apparatus with a die cavity having an internal diameter of about57 mm. The test volume of the particulate sample is equivalent to thevolume which would be occupied by 1.95 g./cm² of 0.85 to 0.425 mm sandin the test cell A steel plunger or piston applies pressure to theparticulate material inside the cavity at a rate of 1 minute to achievethe test pressure, for example about 69 MPa, and 2 minutes at totalpressure, after which the pressure is released.

Subsequently, the sample is screened between screens having openings ofapproximately 0.85, 0.425 or 0.297 mm, for 10 minutes on a screenvibrator such as a ROTAP type screen vibrator available from the Taylordivision of the Combustion Engineering Company of Ohio, and thepercentage of fines less than 0.425 mm in largest dimension is recorded.More crush resistant particulate materials exhibit minimal weightpercent fines produced in the crush resistance test.

Alternative embodiments of skid-resistant surfaces of the presentinvention include sheet materials for application to road surfaces andalso lane marking lines. The skid-resistant spheroids are dropped orcascaded either simultaneously or sequentially with retroreflectivematerials onto wet paint or hot thermoplastic materials, as described inU.S. Pat. No. 3,849,351, incorporated herein by reference. In each ofthese alternative embodiments, a matrix is present and serves to holdthe crush resistant particles in a partially embedded and partiallyprotruding orientation which allows the particles to increase frictionon the exposed surface. Alternative coating compositions which serve asa matrix and include the skid-resistant spherical particles describedherein are also contemplated as included within the scope of the presentinvention.

The ceramic particles could be incorporated into thermoplastic resinmarking systems to impart more durable skid-resistance to markings madeusing this type of composition.

In addition to the pavement marking sheet or tape constructionpreviously described, other tape constructions can be made whichincorporate and benefit from the use of the above-described ceramicparticles. In particular, the use of a polyurethane topcoat is notlimited to the composition previously disclosed, but could include anyof a variety of aliphatic compositions such as those described bySengupta, Ethen, and Jordan in their patent applications, U.S. Pat. No.744,494, filed June 13, 1985, U.S. Pat. No. 4,388,359 and U.S. Pat. No.4,490,432, respectively. Use of particles in conjunction with epoxycoatings and foil backings such as is described in Tung and Frost (U.S.Pat. No. 4,248,932) is also possible.

Possible modifications or equivalents to this invention include the useof ceramic anti-skid particles in tape constructons where the firstlayer 12 is replaced by aluminum foil, an impregnated non-woven scrim,or eliminated altogether. The aliphatic polyurethane of the topcoatcould be replaced with vinyl, epoxy resin, polyester resin, or polyamideresin.

In addition to the use of these particles in tape constructions, use indurable paint compositions is also envisioned. The durable paints couldbe two component systems such as epoxies or polyurethanes or may also bedrying and curing type paints, such as thermoplastic polyurethanes,alkyds, acrylics, polyesters, or equivalents.

EXAMPLE 1

White anti-skid particles were prepared as follows:

A dry blend of the following materials, having particle sizes which passthrough screen openings of about 45 micrometers, were mixed for about 3minutes in an Eirich Machine Inc. RV02 Mix Pelletizer:

    ______________________________________                                        Wt %      Material                                                            ______________________________________                                        50.0      Calcined Alumina (A-2, from Alcoa                                             Alumina Co., Bauxite, Arkansas)                                     2.5       Bentonite Clay (VOLCLAY 200, from                                             American Colloid Co., Skokie, IL)                                   47.5      NEPHELINE SYENITE (APEX 710, from                                             Kraft Chemical Co., Chicago, IL)                                    ______________________________________                                    

15 wt. % water was added over a period of one minute. The water wasdispersed throughout the mixture by mixing for approximately anadditional 15 minutes with the Eirich machine cylindrical containerrotating at about 66 rpm and the impeller rotating at about 2230 rpm.The impeller rotation was subsequently reduced to about 1115 rpm forabout 20 minutes. The result was agglomerated particles predominatelyhaving a diameter of from about 0.85 to about 0.425 mm. The particleswere dried for about hours at approximately 125° C.

A mixture of 86 wt. % dried agglomerated particles and 14 wt. % fine(ie. less than about 45 micrometers) aluminum hydroxide (SolemIndustries, Norcross, Ga. was fed into a tube kiln rotating at about 3rpm, inclined at about a 2 degree angle to horizontal, and operated at afiring temperature of about 1315° C. The kiln length was about 213 cmand kiln diameter was about 14 cm. The fired particles were subsequentlyscreened to remove any excess parting agent. The screening processproduced particles primarily having a diameter from about 0.85 to about0.425 mm in diameter. The particles were white and had a specificgravity of about 3.15 and a loose bulk density of about 1.6 g/cc. Theparticles showed less than 15% wt. fines could be separated after 69 MPapressure (crush resistance test).

Other properties were generally consistent with the particles describedin U.S. Pat. No. 4,680,230.

EXAMPLE 2

Brown anti-skid particles were prepared as follows:

A dry blend of the following materials, having particle sizes which passthrough screen openings of about 45 micrometers, were mixed for about 3minutes in an Eirich Machine Inc. RV02 Mix Pelletizer:

    ______________________________________                                        Wt %       Material                                                           ______________________________________                                        50.0       Calcined Alumina (A-2 , from Alcoa                                            Alumina Co., Bauxite, Arkansas)                                    2.5        Bentonite Clay (VOLCLAY 200, from                                             American Colloid Co., Skokie , IL)                                 47.5       NEPHELINE SYENITE (Kylo-LR                                                    Minnesota Mining and                                                          Manufacturing, St. Paul, Minnesota)                                ______________________________________                                    

16.1 wt. % water was added over a period of 45 seconds. The water wasdispersed throughout the mixture by mixing for approximately anadditional 5 minutes with the Eirich machine cylindrical containerrotating at about 66 rpm and the impeller rotating at about 2230 rpm.The impeller rotation was subsequently reduced to about 1115 rpm forabout 3 minutes. The result was agglomerated particles predominatelyhaving a diameter of from about 0 85 to about 0.425 mm. The particleswere dried for about 16 hours at approximately 125° C.

A mixture of 80 wt. % dried agglomerated particles and 20 wt. % coarse(ie. less than about 150 micrometers but more than 45 micrometers)calcined alumina (A-2 from Alcoa Alumina Co.) was fed into a tube kilnrotating at about 2 rpm, inclined at about a 1 degree angle tohorizontal, and operated at a firing temperature of about 1206° C. Thekiln length was about 213 cm and kiln diameter was about 14 cm. Thefired particles were subsequently screened to remove any excess alumina.By excess alumina is meant alumina which failed to associate with aparticle. The screening process produced particles primarily having adiameter from about 0 85 to about 0.425 mm in diameter The particleswere brown and had a specific gravity of about 2.80 and a loose bulkdensity of about 1.29 g/cc. Less than 22% particles could be screened at69 MPa.

Other properties were generally consistent with the particles describedin U.S. Pat. No. 4,680,230. Coarse alumina was attached onto the surfaceof the particles to form a rough surface.

EXAMPLE 3

A pavement marking tape of this invention was constructed by coating aliquid polyurethane coating onto a calendered acrylonitrile rubbersheet, dropping on ceramic spheroids of Example 1 above, which becameembedded in the coating layer. Subsequently, the liquid polyurethanecoating was cured to a hard, dry film. In particular, an acrylonitrilerubber sheet with the following composition was made by mixing theingredients together in approximately the following proportions in aninternal mixer (a Farrel-Banbury mixer) at about 50 rpm and about 135°C. for about 5 minutes:

    ______________________________________                                        Acrylonitrile-butadiene non-                                                                          100    parts                                          crosslinked elastomer precursor                                               ("Hycar 1022" supplied by B. F.                                               Goodrich)                                                                     Chlorinated paraffin    70     parts                                          ("Chlorowax 70-S" supplied by                                                 Diamond Shamrock)                                                             Chlorinated paraffin    5      parts                                          ("Chlorowax 40")                                                              Polyester fibers ("minifibers"                                                                        10     parts                                          1/4 × 3 denier, supplied by                                             Minifiber, Inc; Johnson City, TN                                              Polyethylene Fibers (fibers of                                                                        20     parts                                          high-density pol-ethylene having                                              molecular weight ranging between                                              30,000 and 150,000                                                            Titanium dioxide pigment                                                                              130    parts                                          Talc platelet filler particles                                                                        100    parts                                          averaging 2 micrometers in                                                    size and having a surface area                                                of 25 square meters per gram                                                  Transparent glass microspheres                                                                        280    parts                                          averaging about 100 micrometers                                               in diameter and having an index                                               of refraction of 1.5                                                          Spherical silica reinforcing                                                                          20     parts                                          filler ("Hisil 233" supplied by                                               PPG Industries)                                                               Stearic acid release agent                                                                            3.5    parts                                          Ultramarine Blue        0.5    parts                                          Chelator ("Vanstay SC" supplied                                                                       0.5    parts                                          by R. T. Vanderbilt, Inc.)                                                    ______________________________________                                    

After removal from the Banbury mixer, the mixture of materials wasmilled to a thickness of about 12.7 mm on a two roll rubber mill. Then,thickness was further reduced to about 1.3 mm in a calender stackoperating at temperature of about 85° C. to about 90° C. The mixture ofmaterials traveled between the rollers at a rate of about 13.7 to about15.3 meters per minute The calendered web was wound into rolls with apolyethylene film interliner

Sheets of the above web (referred to as a "premix") were unwound fromthe roll and coated with a solution of a liquid or uncured polyurethaneresin mixture of approximately the following composition:

    ______________________________________                                        Scotchlite 4430 polyurethane coating                                                                    50    parts                                         (Minnesota Mining and Manufacturing                                           Company, St. Paul, MN)                                                        EXPO3 White (TiO.sub.2) pigment                                                                         20    parts                                         Dispersion (Harwick Chemical                                                  Corp.; Akron, Ohio)                                                           4430B Catalyst (Zinc Hexogen)                                                                           0.5   parts                                         (Minnesota Mining and Manufacturing                                           Company, St. Paul, MN)                                                        ______________________________________                                    

The above solution was coated onto the premix or web at a thickness ofabout 0.254 mm and air dried to a tacky state. At ambient conditions,the pre-drying generally required from about 10 to 15 minutes. Thespheroids of Example 1 were distributed over the tacky surface of thepolyurethane at a rate of 48.8 grams/sq m. and the construction wasdried and cured for 10 minutes at 121° C. Retention of the spheroids mayalso be improved by silane treatment.

A polybutadiene rubber pressure sensitive adhesive (specifically, PM7701 from Minnesota Mining and Manufacturing Company, St. Paul, Minn.)was laminated to the backside of the premix completing the productconstruction.

EXAMPLE 4

A pavement-marking tape of this invention was prepared by the method ofExample 3, except that spheroids prepared in Example 2 were substitutedfor the spheroids of Example 1.

EXAMPLE 5

A pavement-marking tape of this invention was prepared by the method ofExample 3, except that spheroids prepared in Example 2 were extensivelytumbled, screened to remove most of the fines, and subsequentlysubstituted for the spheroids of Example 1.

EXAMPLE 6

A pavement-marking tape was prepared, for comparative purposes, bysubstituting sand for the spheroids of Example 1 in a pavement-markingtape prepared by the method of Example 3.

EXAMPLE 7

A pavement-marking tape was prepared, for comparative purposes, bysubstituting alumina for the spheroids of Example 1 in apavement-marking tape prepared by the method of Example 3.

Portions of the products of Examples 3 through 7 were tested forskid-resistance and durability. The portions of each product Weresimilarly exposed to traffic. The comparative results are presented inFIGS. 2 or 3. Each data point represents the anti-skid propertiesobserved after exposure time as tested on a British Skid ResistanceTester.

In FIG. 2, portions of the pavement marking tapes from Example 4 and 6are compared. The tape including spheroids (Example 4) has an initialskid-resistance of about 90 BPN while the tape including sand (Example6) has an initial skid-resistance of about 60 BPN. Both tapes show adecrease in skid-resistance in the first 3 months of exposure to trafficand more constant skid-resistance properties from about 5 months to thetermination of the test at about 22 months.

However, skid-resistance properties of the spheroid embedded tape(Example 4) during the 5 to 22 month period fall generally within therange of about 45 to about 53 BPN while the sand embedded tape (Example6) is within the much lower range of about 18 to 20 BPN. Thus, thespheroid embedded tape (Example 4) is initially more skid-resistant andmaintains a superior level of skid-resistance relative to the sandembedded tape (Example 6). Further, the skid-resistance, for thespherOid embedded tape, (Example 4) during the 5 to 22 month period isapproximately half of the initial skid-resistance, while the sandembedded tape (Example 6) shows only about one-third of the initialskid-resistance. Thus, the spheroid embedded pavement marking tapes aremore durable in skid-resistance in this test than the sand embeddedpavement marking tape. Additionally, more than 80% of the spheroids wereretained on the coated surface in this test.

In FIG. 3, pavement marking tape having embedded spheroids prepared witha fine parting agent, Aluminum Hydroxide less than 45 micrometers(Example 3) is compared to tapes having embedded sand or alumina. Thespheroid tape (Example 3) has an initial skid-resistance of about 77 BPNand drops to about 56 BPN in seven weeks of exposure to traffic. Bycomparison, the sand embedded tape (Example 6) drops from an initialskid-resistance of about 62 BPN to about 48 BPN by seven weeks. Thealumina embedded tape (Example 7) drops from an initial skid-resistanceof about 73 BPN to about 44 BPN by seven weeks. Thus, the spheroidembedded tape of Example 3 exceeds the performance of the comparisontapes of Examples 6 and 7. Further, the alumina embedded tape Example 7although initially nearing the spheroid embedded tape Example 3 inskid-resistance quickly drops in skid-resistance to levels slightlybelow the sand embedded tape. A tape with extensively tumbled particlesfrom Example 5 is also shown and generally duplicates the results fromExample 3.

Colors other than white may be prepared by applying a surface colorcoating to the ceramic spheroids. For example, the preferred particlesmay be colored orange to indicate the presence of a construction workzone, or blue to indicate the presence of a handicapped parking zone, oryellow to indicate a traffic lane boundary, or black to visually blendinto asphalt pavement or contrast with a light colored pavement. A solof about 33% solids sodium silicate in water, having a SiO₂ /Na₂ O ratioof about 2.75 could be added to the spheroids of Example 1 (weight ratioof about 3.8% sol to 96.2% spheroids) and mixed in a Hobart Mixer forabout 1 minute. The particles should be previously pre-heated to 120° C.Color pigment powder may then be added while stirring for approximatelyanother minute. For example, addition of at least about 5% leadmolyldate will result in orange particles, or at least about 1% cobaltoxide will result in blue particles, or at least about 5% yellow ironoxide pigment optionally replaced in part or totally by lead chromatewill result in yellow particles, or at least about 5% carbon black willresult in black particles. The resulting mixture could be dried at 230°C. and the excess pigment removed by screening. By excess pigment ismeant any pigment not adhering to the particles and capable of passingthrough the openings of the screen. Additional coats can be applied toachieve a thicker color coating.

Alternatively, the preferred particles could be colored throughout theceramic portion, by inclusion of various trace amounts of materials inthe ceramic mixture. For example, inclusion of a small amount of ironimpurities causes a brown color, or inclusion of a small amount ofcobalt oxide causes a blue color, or lead antimonate or lead chromatecauses a yellow color. Other colors are available by reviewing the textC. W. Parmelae, Ceramic Glazes (1951) Industrial Publications, Chicago,Ill.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A substrate marking comprising:a matrix layerwith a top surface and comprising a polymeric matrix and a plurality ofdurable, opaque, skid-resistant ceramic particles, having roundedsurfaces and no substantial points, embedded in and protruding from thetop surface, wherein the ceramic particles comprise a fired ceramic madefrom raw materials comprising about: 10-97 parts by weight of mineralparticulate; 3-90 parts by weight alumina; and at least one part byweight binder; wherein the mineral particulate comprises a ceramicmineral which melts below 1300° C.; does not substantially sublime orvolatilize below 1300° C., and vitrifies on cooling; wherein the binderis characterized by adhering the mineral particulate and alumina powdertogether after pelletizing but before firing.
 2. The substrate markingof claim 1 wherein the ceramic particles are spheroids characterized bya Krumbein roundness of at least 0.8.
 3. The substrate marking of claim2, further comprising a base sheet underlying the matrix layer.
 4. Thesubstrate marking of claim 3 wherein the base sheet comprisesacrylonitrile rubber.
 5. The substrate marking of claim 1 wherein theparticles are characterized by a crush resistance at an applied pressureof 69 MPa of less than 22 wt. % particles under 0.425 mm in largestdiameter resulting from an initial sample having a particle sizedistribution ranging from about 0.85 to about 0.42 mm.
 6. The substratemarking of claim 1 wherein the particles are characterized by a fracturetoughness, K_(IC), greater than about 1.5 Mn/m^(3/2).
 7. The substratemarking of claim 1 wherein the matrix layer comprises polyurethane. 8.The substrate marking of claim 1, wherein the mineral particulate isselected from the group consisting of nepheline syenite, basalt, shale,feldspar, argillite, and pyroxene.
 9. The substrate marking of claim 1,wherein the mineral particulate is nepheline syenite.
 10. The substratemarking of claim 1, wherein the binder is selected from the groupconsisting of bentonite, sodium silicate, cellulose gum, and sodiumlignosulphonate.
 11. A substrate marking comprising:a matrix layer witha top surface and comprising a polymeric matrix and a plurality ofdurable, opaque, skid-resistant ceramic particles, having roundedsurfaces and no substantial points, embedded in and protruding from thetop surface, wherein the ceramic particles comprise a fired ceramic madefrom raw materials comprising about: 10-97 parts by weight of a mineralparticulate; 3-90 parts by weight alumina; and at least one part byweight of a binder; wherein the mineral particulate comprises a ceramicmineral which melts below 1300° C.; does not substantially sublime orvolatilize below 1300° C.; and vitrifies on cooling; wherein the binderis characterized by adhering the mineral particulate and alumina powdertogether after pelletizing but before firing; and wherein the ceramicparticles are characterized by an outer region which is rich in aluminaconcentration as compared to the region near the center of theparticles.